The development of high energy battery systems requires, among other things, the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials such as lithium or the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes, to turn to the investigation of nonaqueous electrolyte systems.
The term "nonaqueous electrolyte" as used herein refers to an electrolyte which is composed of a solute, such as, for example, a metal salt or a complex salt of Group I-A, Group II-A or Group III elements of the Periodic Table, dissolved in an appropriate nonaqueous solvent. The term "Periodic Table" as used herein refers to the Periodic Table of Elements.
To one of ordinary skill in the art to which this invention relates, selection of a solute is relatively straightforward. However, the selection of a suitable solvent has been troublesome. The ideal battery electrolyte should comprise a solvent-solute pair having a long liquid range, high ionic conductivity and suitable stability. A long liquid range, i.e., high boiling point and low freezing point, is essential if the battery is to operate within a wide range of temperatures. High ionic conductivity is necessary if the battery is to have high rate capability. Stability is necessary with the electrode materials, the materials of cell construction and the products of the cell reaction to provide a long shelf life and a long operating life.
It has been disclosed in the literature that certain materials are capable of acting both as electrolyte carriers, i.e., as solvent for the electrolyte salt, and as the active cathode for an nonaqueous eletrochemical cell. U.S. Pat. Nos. 3,567,515 and 3,578,500 each disclose that liquid sulfur dioxide or solutions of sulfur dioxide and a co-solvent will perform this dual function in nonaqueous electrochemical cells. Other cathode-electrolyte systems are disclosed in U.S. Pat. Nos. 4,328,289 issued May 4, 1982 to Zupancic et al, 4,264,687 issed Apr. 28, 1981 to Dey et al, 4,012,564 issued Mar. 15, 1977 to Auborn, 3,998,657 issued to Auborn et al on Dec. 21, 1976, and 3,926,669 issued to Auborn on Dec. 16, 1975.
U.S. Pat. No. 4,400,453 to G. E. Blomgren et al, issued Aug. 23, 1983, assigned to the Union Carbide Corporation, discloses a nonaqueous electrochemical cell comprising an anode, a cathode current collector and a cathode-electrolyte, the cathode-electrolyte comprising a solution of an ionically conductive solute dissolved in an active cathode depolarizer wherein the active cathode depolarizer consists of a liquid oxyhalide of an element of Group V or Group VI of the Periodic Table. Oxyhalides can be used effectively as a component part of a cathode-electrolyte in conjunction with an active metal anode such as lithium to produce a good high energy density cell. A drawback of oxyhalide cathode-electrolytes has been observed if the cell is stored for a prolonged period of about three days or longer. Passivation of the anode or drop in the cell voltage at the beginning of discharge has been observed in such stored cells.
One of the primary objects of this invention is to substantially reduce the passivation of the active metal anode in oxyhalide cathode-electrolyte cells.
One approach to the problem of anode passivation is described in U.S. Pat. No. 3,993,501 issued Nov. 23, 1976 to Kalnoki-Kis and assigned to Union Carbide Corporation. The '501 patent describes the use of a vinyl polymer film on the surface of the anode where such anode is in contact with liquid cathode-electrolyte. U.S. Pat. No. 4,170,693 issued Oct. 9, 1979 to Catanzarite discloses the use of cyanoacrylate organic compounds coated on the active metal anode. While the Kalnoki-Kis and Cantanzarite approaches to cell passivation appear to be workable, particularly the Cantanzarite approach suffers the drawback of permitting substantial cell self-discharge during storage. Self-discharge and cell passivation are particularly bothersome in such high performance, remote access applications as power supplies for cardiac pacemakers. Thus, the present invention is intended to substantially reduce cell passivation and simultaneously to mimimize self-discharge, particularly as they occur in liquid cathode-electrolyte cells. Furthermore, cell reliability and safety are enhanced by the practice of the instant invention because the anode coating herein described appears to be more resistant to degradation or dissolution than other materials disclosed in the art.